Introduction to Fungi, Third Edition

246 ASCOMYCOTA (ASCOMYCETES)
forming a gymnothecium (Gr. gymnos ¼ naked)
and the asci can be seen through the network
(Fig. 11.8). Gymnothecia are also seen in
Myxotrichum (Fig. 11.10) and Ctenomyces (Fig. 11.5)
where certain peridial hyphae extend as hooked
hairs. In most species of Aspergillus and Penicillium
which possess ascocarps, the asci are enclosed
in a globose fructification with no special
opening to the outside. Such ascocarps are
termed cleistocarps or cleistothecia (Gr.
kleistos ¼ enclosed). A modified cleistothecium
capable of cracking open along a line of weakness
is found in the Erysiphales (powdery
mildews), and this is called a chasmothecium
(Gr. chasma ¼ an open mouth). In the cup fungi
(Pezizales and Helotiales) as well as in many
lichenized ascomycetes the asci are borne in
open saucer-shaped ascocarps, and at maturity
the tips of the asci are freely exposed (see Plates 6
and 7). Such fruit bodies are termed apothecia (Gr.
apo- ¼ away from, separate). The Pyrenomycetes
(e.g. Sphaeriales and Hypocreales) have perithecia
(Gr. peri- ¼ around) which are flask-shaped
fruit bodies opening by a pore or ostiole (see
Fig. 12.1, Sordaria fimicola). The perithecial wall
is formed from sterile cells derived from hyphae
which surrounded the ascogonium during development.
Perithecia are often single, as in Sordaria
and Neurospora, but in some genera they are
embedded in or seated on a mass of tissue
forming a perithecial stroma (for examples,
see Plate 5). The development of pseudothecia
differs from that of perithecia in that the asci
are contained in one or several cavities (locules)
formed within a pre-existing ascostroma (Gr.
stroma ¼ mattress, bed) (Luttrell, 1981). Examples
are Leptosphaeria (Fig. 17.3) and Sporormiella
(Fig. 17.18). Although the structure and development
of perithecia and pseudothecia are essentially
different, the term perithecium is often
loosely applied to both.
8.8 Fossil ascomycetes
Ascomycetes are an ancient group of fungi, and
fossilized structures possibly representing ascocarps
made up of septate, anastomosing hyphae
have been described from the Proterozoic period
about 1 billion years ago (Butterfield, 2005).
Lichen-like associations between fungi and
cyanobacteria or algae may have existed some
600 million years ago (Yuan et al., 2005). What are
believed to be the remains of perithecia have
been reported from beneath the epidermis of
stems and rhizomes of one of the earliest known
land plants, Asteroxylon, in the Rhynie chert of
the Devonian period about 400 million years ago
(Taylor et al., 1999, 2005). Fossil cleistothecia containing
asci and ascospores resembling those
of present-day Trichocomaceae have been
found in coal balls of the Carboniferous age
(Stubblefield & Taylor, 1983; Stubblefield et al.,
1983). Stalked ascocarps with well-preserved
ascospores have been found in amber, the
fossilized resin of a conifer. They have been
assigned to an extant genus Chaenothecopsis
(Mycocaliciaceae). Their close resemblance of
present-day species which are also associated
with resin indicate little evolutionary change
during the past 20 million years (Rikkinen &
Poinar, 2000).
On morphological grounds, Barr (1983)
suggested that the ancestors of Ascomycota
should be sought among the Chytridiomycota.
Confirmation of this view has since been
obtained by comparison of DNA sequence
data. Ascomycota are also closely related to
Basidiomycota, each being a derived monophyletic
group (Bruns et al., 1992; Berbee & Taylor,
2001). Berbee and Taylor (2001) have estimated
that these two groups evolved from a common
ancestor about 600 million years ago, well
before the development of vascular terrestrial
plants.
8.9 Scientific and economic
significance of ascomycetes
The study of ascomycetes is of considerable
scientific importance. Neurospora crassa has been
the subject of intensive genetical research
related to its relatively simple nutrient requirements,
rapid growth, its capacity to produce